Lithium ion secondary battery

ABSTRACT

A lithium ion secondary battery in which a high capacity, as well as a high level of safety, is achieved, by using an oxide material containing Li and Fe for a negative electrode active material. In the lithium ion secondary battery, the negative electrode active material is a mixed phase of LiFeO2 and LiFe5O8 and a material in which the value calculated as the ratio of the height of a diffraction peak belonging to LiFeO2 (200) plane and the height of a diffraction peak belonging to LiFe5O8 (311) plane, which are obtained by X-ray diffraction method, is 0.18 to 20.4.

TECHNICAL FIELD

This invention relates to a lithium ion secondary battery which isexcellent in the energy density characteristics.

BACKGROUND ART

As a power source for an electronic device, a lithium ion secondarybattery is expected to serve as a secondary battery in which downsizingand weight saving are expected. As a negative electrode active materialof such a lithium ion secondary battery, a carbon material such asgraphite (artificial graphite and natural graphite) and amorphous carbonand an alloy material containing silicon, tin or the like as the maincomponent have been studied and practically used.

In recent years, however, as the demand for increasing the energydensity of a battery increases in order to apply a battery to a largeproduct such as an electric vehicle, technical development of a materialwith a high capacity per unit weight has been required. Further, withthe increase in the energy density of a battery, it is required toenhance safety at the same time.

While a lithium ion secondary battery is charged, the potential relativeto Li metal becomes around 0 V with the conventional materials describedabove (the carbon material and the alloy material), and thus there hasbeen a risk that an Li metal dendrite generates if the battery isdeteriorated or overcharged. Accordingly, lithium titanate, in which thepotential during charging is more than 1 V and a dendrite of Li metaldoes not generate, has attracted attention as a new negative electrodematerial.

PTL 1 discloses a technique using a negative electrode material in whichthe potential relative to Li metal is 1 V or more in order to reduce therisk of the generation of an Li metal dendrite during thecharge-discharge cycles. Further, it is suggested that the negativeelectrode material used is an oxide of lithium titanate such asLi_(4+x)Ti₅O₁₂ (x=−1 to 3) having a spinel structure and Li_(2+y)Ti₃O₇(y=−1 to 3) having a ramsdellite structure. PTL 2 discloses a techniqueregarding a discharge capacity exceeding the theoretical capacity ofgraphite, 372 mAh/g, by using a mixture of NaFeO₂ and graphite as thenegative electrode material. It is suggested that the insertion andremoval of Li are easy because NaFeO₂ has a layered rock salt structurelike LiCoO₂ and the like, which are known positive electrode materials.Further, PTL 3 discloses a technique in which about 40 cycles ofcharging and discharging become possible by using LiN(CF₃SO₂)₂ as an Lisalt with LiFe₅O₈ which has been prepared by mixing compounds such asFeOOH and LiOH in an Li/Fe molar ratio of 10/1 to 10/7 and sintering themixture.

Citation List Patent Literature

-   PTL 1: JP-A-2010-153258-   PTL 2: JP-A-2010-218834-   PTL 3: JP-A-11-025977

SUMMARY OF INVENTION Technical Problem

However, in addition to a high level of safety, an increase in thecapacity is required at the same time for a negative electrode activematerial used for a lithium ion secondary battery for an electricvehicle. In addition, because Na has a larger molecular weight than Li,there is a possibility that it is unfavorable for increasing thecapacity per weight. Furthermore, LiPF₆ and LiBF₄ have been generallyused as Li salts of the electrolyte in the conventional Li ionbatteries, and it is desirable that a negative electrode material can becharged and discharged even when LiPF₆ is used instead of LiN(CF₃SO₂)₂,in view of the availability as a product or the like.

An object of this invention is to provide a lithium ion secondarybattery in which the initial charge-discharge efficiency is improved anda high capacity is achieved by using an oxide material containing Li andFe as a negative electrode active material.

Solution to Problem

This invention is characterized in that, in a lithium ion secondarybattery containing a positive electrode and a negative electrode facingeach other through a separator, negative electrode active material is amixed phase of LiFeO₂ and LiFe₅O₈ and comprises a material in which thevalue calculated as the ratio of the height of a peak belonging toLiFeO₂ (200) plane and the height of a peak belonging to LiFe₅O₈ (311)plane, which are obtained by X-ray diffraction method, is 0.18 to 20.4.

Advantageous Effects

In this invention, a mixture of oxide materials containing Li and Fe inwhich the main components of the oxides are represented by LiFeO₂ orLiFe₅O₈ is used as the negative electrode active material, and thus itis possible to provide a lithium ion secondary battery in which theinitial charge-discharge efficiency of the negative electrode materialis increased to more than 77% and a high level of safety and anincreased capacity are both ensured.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows a schematic cross-sectional view of a model battery towhich this invention is applied.

FIG. 2 shows results of X-ray diffraction regarding Example 3.

FIG. 3 shows results of X-ray diffraction regarding Comparative Example1.

DESCRIPTION OF EMBODIMENTS (Production of LiFeO₂ and LiFe₅O₈ Oxides)

The mixture of LiFeO₂ and LiFe₅O₈ oxides was produced by the followingprocedure. Lithium hydroxide monohydrate (manufactured by Wako PureChemical Industries, Ltd.) was used as the Li raw material and ironoxyhydroxide (manufactured by Kojundo Chemical Laboratory Co., Ltd.) oriron (III) oxide (Fe₂O₃) was used as the Fe raw material. First, the rawmaterial compounds were mixed in a certain Li and Fe molar ratio and putinto a sealed-type sample reactor (manufactured by SAN-AI Kagaku Co.Ltd.) with distilled water (manufactured by Wako Pure ChemicalIndustries, Ltd.). Then, the reactor was placed in an electric furnaceand kept at 200° C. for a certain time to conduct hydrothermal reaction.The material treated was washed with distilled water for several times,separated from the solution by filtration and dried at 80° C. for fivehours to produce the oxide mixture.

The synthesis condition of the material regarding this inventiondescribed above is an example and the condition is not limited by thenumerical values described. For example, the sample may be dried afterthe filtration under a reduced pressure condition using a vacuum dryeror the like.

(Identification of Crystal Phase)

The crystal state of the sample prepared was identified using awide-angle X-ray diffraction apparatus (manufactured by RigakuCorporation, RU200B). The measurement condition for identifying thecrystals is as follows.

The X-ray source was Cu and the output power thereof was set to be 50 kVand 150 mA. A concentration-method optical system with a monochromatorwas used, and a divergence slit of 1.0 deg, a receiving slit of 0.3 mmand a scattering slit of 1.0 deg were selected. The scan axis of X-raydiffraction was 2θ/θ interlock system and the measurement was conductedin the range of 30≦2θ≦50 deg by continuous scanning under the conditionof a scanning speed of 2.0 deg/min and sampling of 0.02 deg.

Regarding the crystal identification, the crystals precipitated in thematerial were identified using ICDD data, which is an X-ray diffractionstandard data set.

Examples of this invention, are shown below.

Example 1

Lithium hydroxide monohydrate (manufactured by Wako Pure ChemicalIndustries, Ltd.) was used as the Li raw material and α-ironoxyhydroxide (manufactured by Kojundo Chemical Laboratory Co., Ltd.) wasused as the Fe raw material. First, the raw material compounds weremixed in an Li and Fe molar ratio of 3.0/1 and put into a sealed-typesample reactor (manufactured by SAN-AI Kagaku Co. Ltd.) with distilledwater (manufactured by Wako Pure Chemical Industries, Ltd.). Then, thereactor was placed in an electric furnace and kept at 200° C. for 20hours to conduct hydrothermal reaction. The material treated was washedwith distilled water for several times, separated from the solution byfiltration and dried at 80° C. for five hours and the material obtainedwas subjected to X-ray diffraction measurement. As a result ofidentification of the crystals precipitated in the material using TCDDdata, which is an X-ray diffraction standard data set, it was confirmedthat LiFeO₂ and LiFe₅O₈ were contained.

Example 2

Lithium hydroxide monohydrate (manufactured by Wako Pure ChemicalIndustries, Ltd.) was used as the Li raw material and γ-iron (III) oxide“γ-Fe₂O₃” (manufactured by Kojundo Chemical Laboratory Co., Ltd.) wasused as, the Fe raw material. First, the raw material compounds weremixed in an Li and Fe molar ratio of 1.5/1 and put into a sealed-typesample reactor (manufactured by SAN-AI Kagaku Co. Ltd.) with distilledwater (manufactured by Wako Pure Chemical Industries, Ltd.). Then, thereactor was placed in an electric furnace and kept at 200° C. for 20hours to conduct hydrothermal reaction. The material treated was washedwith distilled, water for several times, separated from the solution byfiltration and dried at 80° C. for five hours. As a result of X-raydiffraction of the material prepared and identification of the crystalsprecipitated in the material using ICDD data, which is a standard dataset, it was confirmed that LiFeO₂ and LiFe₅O₈ were contained.

Example 3

Hydrothermal synthesis was conducted in accordance with Example 2 exceptthat the raw material compounds were mixed in an Li and Fe molar ratioof 2.5/1. As a result of X-ray diffraction of the material prepared andidentification of the crystals precipitated in the material using ICDDdata, which is a standard data set, it was confirmed that LiFeO₂ andLiFe₅O₈ were contained.

Example 4

Hydrothermal synthesis was conducted in accordance with Example 1 exceptthat γ-iron oxyhydroxide (manufactured by Kojundo Chemical LaboratoryCo., Ltd.) was used as the Fe raw material. As a result of X-raydiffraction of the material prepared and identification of the crystalsprecipitated in the material using ICDD data, which is a standard dataset, was confirmed that LiFeO₂ and LiFe₅O₈ were contained.

Example 5

Hydrothermal synthesis was conducted in accordance with Example 4 exceptthat the raw material compounds were mixed in an Li and Fe molar ratioof 5.0/1. As a result of X-ray diffraction of the material prepared andidentification of the crystals precipitated in the material using TCDDdata, which is a standard data set, it was confirmed that LiFeO₂ andLiFe₅O₈ were contained.

Example 6

Lithium hydroxide monohydrate (manufactured by Wako Pure ChemicalIndustries, Ltd.) was used as the Li raw material and γ-iron (III) oxide“γ-Fe₂O₃” (manufactured by Kojundo Chemical Laboratory Co., Ltd.) wasused as the Fe raw material. First, the raw material compounds weremixed in an Li and Fe molar ratio of 3.0/1 and put into a sealed-typesample reactor (manufactured by SAN-AT Kagaku Co. Ltd.) with distilledwater (manufactured by Wako Pure Chemical Industries, Ltd.). Then, thereactor was placed in an electric furnace and kept at 200° C. for 10hours to conduct hydrothermal reaction. The material treated was washedwith distilled water for several times, separated from the solution byfiltration and dried at 80° C. for five hours. As a result of X-raydiffraction of the material prepared and identification of the crystalsprecipitated in the material using ICDD data, which is a standard dataset, it was confirmed that LiFeO₂ and LiFe₅O₈ were contained.

Example 7

Hydrothermal synthesis was conducted in accordance with Example 6 exceptthat γ-iron oxyhydroxide (manufactured by Kojundo Chemical LaboratoryCo., Ltd.) was used as the Fe raw material. As a result of X-raydiffraction of the material prepared and identification of the crystalsprecipitated in the material using ICDD data, which is a standard dataset, it was confirmed that LiFeO₂ and LiFe₅O₈ were contained.

Example 8

Hydrothermal synthesis was conducted in accordance with Example 2 exceptthat the raw material compounds were mixed in an Li and Fe molar ratioof 5.0/1. As a result of X-ray diffraction of the material prepared andidentification of the crystals precipitated in the material using ICDDdata, which is a standard data set, it was confirmed that LiFeO₂ andLiFe₅O₈ were contained.

Example 9

Hydrothermal synthesis was conducted in accordance with Example 2 exceptthat the raw material compounds were mixed in an Li and Fe molar ratioof 3.0/1. As a result of X-ray diffraction of the material prepared andidentification of the crystals precipitated in the material using ICDDdata, which is a standard data set, it was confirmed that LiFeO₂ andLiFe₅O₈ were contained.

Example 10

Hydrothermal synthesis was conducted in accordance with Example 7 exceptthat the raw material compounds were mixed in an Li and Fe molar ratioof 1.5/1. As a result of X-ray diffraction of the material prepared andidentification of the crystals precipitated in the material using ICDDdata, which is a standard data set, it was confirmed that LiFeO₂ andLiFe₅O₈ were contained.

Example 11

Hydrothermal synthesis was conducted in accordance with Example 10except that the raw material compounds were mixed in an Li and Fe molarratio of 2.5/1. As a result of X-ray, diffraction of the materialprepared and identification of the crystals precipitated in the materialusing ICDD data, which is a standard data set, it was confirmed thatLiFeO₂ and LiFe₅O₈ were contained.

Example 12

Hydrothermal synthesis was conducted in accordance with Example 8 exceptthat the hydrothermal synthesis time was changed to five hours. As aresult of X-ray diffraction of the material prepared and identificationof the crystals precipitated in the material using ICDD data, which is astandard data set, it was confirmed that LiFeO₂ and LiFe₅O₈ werecontained.

Example 13

Hydrothermal synthesis was conducted in accordance with Example 8 exceptthat the thermal synthesis time was changed to 10 hours. As a result ofX-ray diffraction of the material prepared and identification of thecrystals precipitated in the material using ICDD data, which is astandard data set, it was confirmed that LiFeO₂ and LiFe₅O₈ werecontained.

Example 14

Hydrothermal synthesis was conducted in accordance with Example 7 exceptthat γ-iron oxyhydroxide (manufactured by Kojundo Chemical LaboratoryCo., Ltd.) and γ-iron (III) oxide “γ-Fe₂O₃” (manufactured by KojundoChemical Laboratory Co., Ltd.) as the Fe raw materials were mixed in amolar ratio of 2/1 and this mixture was incorporated in an Li and Femolar ratio of 3.0/1. As a result of X-ray diffraction of the materialprepared and identification of the crystals precipitated in the materialusing ICDD data, which is a standard data set, it was confirmed thatLiFeO₂ and LiFe₅O₈ were contained.

Example 15

Hydrothermal synthesis was conducted in accordance with Example 7 exceptthat γ-iron oxyhydroxide (manufactured by Kojundo Chemical LaboratoryCo., Ltd.) and γ-iron (III) oxide “γ-Fe₂O₃” (manufactured by KojundoChemical Laboratory Co., Ltd.) as the Fe raw materials were mixed in amolar ratio of 2/1 and this mixture was incorporated in an Li and Femolar ratio of 5.0/1. As a result of X-ray diffraction of the materialprepared and identification of the crystals precipitated in the materialusing ICDD data, which is a standard data set, it was confirmed thatLiFeO₂ and LiFe₅O₈ were contained.

Comparative Example 1

Hydrothermal synthesis was conducted in accordance with Example 13except that the raw material compounds were mixed in an Li and Fe molarratio of 1/1. As a result of X-ray diffraction of the material preparedand identification of the crystals precipitated in the material usingICDD data, which is a standard data set, it was confirmed that LiFeO₂and LiFe₅O₈ were contained.

Comparative Example 2

Hydrothermal synthesis was conducted in accordance with Example 8 exceptthat the raw material compounds were mixed in an Li and Fe molar ratioof 0.75/1. As a result of X-ray diffraction of the material prepared andidentification of the crystals precipitated in the material using ICDDdata, which is a standard data set, it was confirmed that LiFeO₂ andLiFe₅O₈ were contained.

Comparative Example 3

γ-Iron (III) oxide “γ-Fe₂O₃” (manufactured by Kojundo ChemicalLaboratory Co., Ltd.) was directly used.

Comparative Example 4

γ-Iron oxyhydroxide (manufactured by Kojundo Chemical Laboratory Co.,Ltd.) was directly used.

Comparative Example 5

LiFeO₂ which was prepared in accordance with PTL 1 (JP-A-2010-153258)was used as the negative electrode active material. Specifically, it wasprepared by mixing lithium carbonate (manufactured by Kojundo ChemicalLaboratory Co., Ltd.) and γ-iron (III) oxide “γ-Fe₂O₃” (manufactured byKojundo Chemical Laboratory Co., Ltd.) in the same amount as a molnumber, temporarily powder-compacting to obtain pellets and calcining at900° C. for 12 hours.

The summary of the above conditions is shown in Table 1. Table 1 showsthe kinds of the Fe raw material regarding this invention, the chargedcompositions and the synthesis conditions thereof.

TABLE 1 Li/Fe Kind of Fe Raw Charged Synthetic Treatment Material MolarRatio Method Time Example 1 αFeOOH 3/1 Hydrothermal 20 h SynthesisExample 2 γFe₂O₃ 1.5/1   Hydrothermal 20 h Synthesis Example 3 γFe₂O₃2.5/1   Hydrothermal 20 h Synthesis Example 4 γFeOOH 3/1 Hydrothermal 20h Synthesis Example 5 γFeOOH 5/1 Hydrothermal 20 h Synthesis Example 6γFe₂O₃ 3/1 Hydrothermal 10 h Synthesis Example 7 γFeOOH 3/1 Hydrothermal10 h Synthesis Example 8 γFe₂O₃ 5/1 Hydrothermal 20 h Synthesis Example9 γFe₂O₃ 3/1 Hydrothermal 20 h Synthesis Example 10 γFeOOH 1.5/1  Hydrothermal 20 h Synthesis Example 11 γFeOOH 2.5/1   Hydrothermal 20 hSynthesis Example 12 γFe₂O₃ 5/1 Hydrothermal  5 h Synthesis Example 13γFe₂O₃ 5/1 Hydrothermal 10 h Synthesis Example 14 γFe₂O₃ + γFeOOH 3/1Hydrothermal 10 h Synthesis Example 15 γFe₂O₃ + γFeOOH 5/1 Hydrothermal10 h Synthesis Comparative γFe₂O₃ 1/1 Hydrothermal 10 h Example 1Synthesis Comparative γFe₂O₃ 0.75/1   Hydrothermal 20 h Example 2Synthesis Comparative γFe₂O₃ Not Treated Not Treated Not Treated Example3 Comparative γFeOOH Not Treated Not Treated Not Treated Example 4Comparative γFe₂O₃ 1/1 Calcination 12 h (900° C.) Example 5 inAtmosphere

(Peak Ratio Calculation)

The ratio of LiFeO₂ and LiFe₅O₈ was calculated as the ratio ofdiffraction peak heights obtained by the above XRD diffraction method.

The height of a peak belonging to LiFeO₂ (200) plane and the value of apeak belonging to LiFe₅O₈ (311) plane were used.

The test calculation of the peak ratio is shown by the Formula 1.

Peak Ratio=Peak Value of LiFeO₂ (002) Plane/Peak Value of LiFe₅O₈ (311)Plane  (Formula 1)

The XRD pattern of the material shown in Example 3 is shown in FIG. 2.As a result of calculation by (Formula 1) based on this result, the peakratio was 0.23.

The XRD pattern of the material shown in Comparative Example 1 is shownin FIG. 3. As a result of calculation by (Formula 1) based on thisresult, the peak ratio was 0.16.

The peak ratios of Examples 1 to 15 and Comparative Examples 1, 2 and 5are shown in Table 2. Table 2 shows the comparative results of the peakratios and the initial charge-discharge efficiencies regarding thisinvention.

TABLE 2 Initial Efficiency XRD Peak Ratio (%) Example 1 20.40 80.1Example 2 0.18 77.0 Example 3 0.23 77.6 Example 4 0.74 83.3 Example 58.47 83.9 Example 6 1.01 82.2 Example 7 1.04 82.0 Example 8 1.50 83.6Example 9 0.25 78.9 Example 10 0.29 78.7 Example 11 0.47 78.9 Example 121.54 80.8 Example 13 2.20 82.8 Example 14 0.79 79.6 Example 15 1.67 80.3Comparative 0.16 74.7 Example 1 Comparative 0.03 72.5 Example 2Comparative — 71.0 Example 3 Comparative — 75.5 Example 4 Comparative29.94 51.4 Example 5

(Evaluation of Charge-Discharge Efficiency)

Next, the charge-discharge evaluation of these negative electrodematerials is explained.

FIG. 1 is a schematic view showing an example of a model battery.Explanation below is made referring to this figure.

In this figure, a negative electrode layer containing a negativeelectrode active material and a conductive adjuvant is formed on thesurface of a negative electrode current collector and they constitute anegative electrode 13. Further, for the evaluation this time, metal Lifoil was used as a counter electrode 11.

Specifically, 80% by mass of a negative electrode powder (the negativeelectrode active material 2), 10% by mass of carbon black (theconductive adjuvant 3) and 10% by mass of a binder were mixed and normalmethylpyrrolidon was added to produce a paste having a viscosity of 15Pa·s (25° C.). The paste produced was coated on copper foil of thenegative electrode current collector with a doctor blade and dried andthus the negative electrode layer was produced. The negative electrode13 was produced by punching out the negative electrode layer and thenegative electrode current collector together.

Then, as shown in FIG. 1, a separator 12 was inserted between thecounter electrode 11 (metal Li foil was used this time) and the negativeelectrode 13 and they were placed in a battery case 14 of a coinbattery. A gasket 15 was set and then a top cover 16 was provided. Acoin-type cell was thus produced.

Here, a mixed solvent of ethylene carbonate (EC) and ethyl methylcarbonate (EMC) containing 1 mol of LiPF₆/(EC:EMC=1:2) was used as theelectrolyte. Regarding the charge-discharge characteristics of the modelbattery, also using TSCAT 3000 (manufactured by Toyo system Co., Ltd.),the battery charge-discharge evaluation was conducted at a currentdensity of 0.3 mA/cm² by charging and discharging within the range of3.0 to 0.1 V (vs. Li/Li⁺) and the initial charge capacity (mAh/g) anddischarge capacity (mAh/g) per weight of the active material containedin the electrode were measured. Further, the initial charge-dischargeefficiency was calculated by the (Formula 2) shown below.

Initial Efficiency (%)=(Initial Discharge Capacity/Initial ChargeCapacity)×100  (Formula 2)

The calculation results of the initial efficiencies of Examples 1 to 15and Comparative Examples 1 to 5 are shown in Table 2.

As a result, it was confirmed that the initial charge-dischargeefficiencies were as high as 77% or more with the materials having peakratios of 0.18 to 20.4.

INDUSTRIAL APPLICABILITY

The negative electrode active material obtained in this invention has ahigher capacity per weight than the conventionally-used carbon materialand prevents the generation of a dendrite due to the excellent chargepotential. Thus, it is expected that the negative electrode activematerial is applied to a power source of a mobile object or a stationarypower storage, which requires a large lithium ion secondary batteryexcellent in safety.

REFERENCE SIGN LIST

-   -   11: Counter Electrode    -   12: Separator    -   13: Negative Electrode    -   14: Battery Case    -   15: Gasket    -   16: Top Cover

1. A lithium ion secondary battery comprising a positive electrode and anegative electrode facing each other through a separator, which ischaracterized in that a negative electrode active material is a mixedphase of LiFeO₂ and LiFe₅O₈ obtained by hydrothermal synthesis andcomprises a material in which a value calculated as the ratio of theheight of a diffraction peak belonging to LiFeO₂ (200) plane and theheight of a diffraction peak belonging to LiFe₅O₈ (311) plane, which areobtained by X-ray diffraction method, is 0.18 to 20.4.
 2. The lithiumion secondary battery of claim 1, which is characterized in that thenegative electrode active material is a material showing a Coulombefficiency of initial charging and discharging at 0.1 to 3 V of 77% ormore.
 3. The lithium ion secondary battery of claim 1, which ischaracterized in that the negative electrode active material is obtainedby hydrothermal synthesis using Fe2O3 as a raw material.
 4. A method forproducing a negative electrode active material for a lithium ionsecondary battery containing LiFeO2 and LiFe5O8, which is characterizedby using Fe2O3 as a raw material and producing the negative electrodeactive material by hydrothermal synthesis.
 5. The method for producing anegative electrode active material of claim 4, which is characterized inthat the Li/Fe ratio of the raw material is 1.5 to 5.0.